Sodium stannate additive to improve the durability of PEMS for H2/air fuel cells

ABSTRACT

An ion conducting membrane for fuel cell applications includes an ion conducting polymer and a tin-containing compound at least partially dispersed within the ion conducting polymer. The ion conducting membranes exhibit improved performance over membranes not incorporating such tin-containing compounds.

TECHNICAL FIELD

The present invention relates to ion conducting membranes for fuel cellapplications.

BACKGROUND

Fuel cells are used as an electrical power source in many applications.In particular, fuel cells are proposed for use in automobiles to replaceinternal combustion engines. A commonly used fuel cell design uses asolid polymer electrolyte (“SPE”) membrane or proton exchange membrane(“PEM”) to provide ion transport between the anode and cathode.

In proton exchange membrane type fuel cells, hydrogen is supplied to theanode as fuel and oxygen is supplied to the cathode as the oxidant. Theoxygen can either be in pure form (O₂) or air (a mixture of O₂ and N₂).PEM fuel cells typically have a membrane electrode assembly (“MEA”) inwhich a solid polymer membrane has an anode catalyst on one face, and acathode catalyst on the opposite face. The anode and cathode layers of atypical PEM fuel cell are formed of porous conductive materials, such aswoven graphite, graphitized sheets, or carbon paper to enable the fuelto disperse over the surface of the membrane facing the fuel supplyelectrode. Each electrode has finely divided catalyst particles (forexample, platinum particles), supported on carbon particles, to promoteoxidation of hydrogen at the anode and reduction of oxygen at thecathode. Protons flow from the anode through the ion conductive polymermembrane to the cathode where they combine with oxygen to form waterwhich is discharged from the cell. Typically, the ion conductive polymermembrane includes a perfluorinated sulfonic acid (PFSA) ionomer.

The MEA is sandwiched between a pair of porous gas diffusion layers(“GDL”), which in turn are sandwiched between a pair of non-porous,electrically conductive elements or plates. The plates function ascurrent collectors for the anode and the cathode, and containappropriate channels and openings formed therein for distributing thefuel cell's gaseous reactants over the surface of respective anode andcathode catalysts. In order to produce electricity efficiently, thepolymer electrolyte membrane of a PEM fuel cell must be thin, chemicallystable, proton transmissive, non-electrically conductive and gasimpermeable. In typical applications, fuel cells are provided in arraysof many individual fuel cell stacks in order to provide high levels ofelectrical power.

One mechanism by which ion conducting polymer membranes degrade is vialoss of fluorine (i.e., fluoride emission) under open circuit voltage(OCV) and dry operating conditions at elevated temperatures. Additivesto PFSA membranes are required to improve fuel cell life, increasemembrane durability and reduce fluoride emissions under theseconditions.

Accordingly, there is a need for improved ion conducting membranes withreduced fluoride emissions.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

The present invention solves one or more problems of the prior art byproviding in at least one embodiment an ion conducting membrane for fuelcell applications. The ion conducting membrane of this embodimentincludes an ion conducting polymer and a tin-containing compound atleast partially dispersed within the ion conducting polymer in asufficient amount to reduce fluoride emissions from the membrane.Moreover, the incorporation of a tin-containing compound advantageouslyincreases membrane life while decreasing electrode voltage degradationin fuel cells operating at open circuit conditions at 95° C. and 50%relative humidity. Additional benefits include reduced cost comparedwith additives presently used to mitigate PFSA-fuel cell ion conductingmembranes.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will become more fullyunderstood from the detailed description and the accompanying drawings,wherein:

FIG. 1 is a schematic illustration of a fuel cell that incorporates anion conducting membrane of one or more embodiments of the invention;

FIG. 2 provides plots of the cell voltage degradation and fluoriderelease rate (“FRR”) versus time for Nafion® 1000 membrane with andwithout sodium stannate;

FIG. 3 provides plots of the relative humidity (“RH”) sweep profile forNafion® 1000 with sodium stannate (in the figure HFR is high frequencyresistance); and

FIG. 4 provides plots of the RH sweep of Nafion® 1000 with and withoutsodium stannate.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Reference will now be made in detail to presently preferredcompositions, embodiments and methods of the present invention, whichconstitute the best modes of practicing the invention presently known tothe inventors. The Figures are not necessarily to scale. However, it isto be understood that the disclosed embodiments are merely exemplary ofthe invention that may be embodied in various and alternative forms.Therefore, specific details disclosed herein are not to be interpretedas limiting, but merely as a representative basis for any aspect of theinvention and/or as a representative basis for teaching one skilled inthe art to variously employ the present invention.

Except in the examples, or where otherwise expressly indicated, allnumerical quantities in this description indicating amounts of materialor conditions of reaction and/or use are to be understood as modified bythe word “about” in describing the broadest scope of the invention.Practice within the numerical limits stated is generally preferred.Also, unless expressly stated to the contrary: percent, “parts of,” andratio values are by weight; the description of a group or class ofmaterials as suitable or preferred for a given purpose in connectionwith the invention implies that mixtures of any two or more of themembers of the group or class are equally suitable or preferred;description of constituents in chemical terms refers to the constituentsat the time of addition to any combination specified in the description,and does not necessarily preclude chemical interactions among theconstituents of a mixture once mixed; the first definition of an acronymor other abbreviation applies to all subsequent uses herein of the sameabbreviation and applies mutatis mutandis to normal grammaticalvariations of the initially defined abbreviation; and, unless expresslystated to the contrary, measurement of a property is determined by thesame technique as previously or later referenced for the same property.

It is also to be understood that this invention is not limited to thespecific embodiments and methods described below, as specific componentsand/or conditions may, of course, vary. Furthermore, the terminologyused herein is used only for the purpose of describing particularembodiments of the present invention and is not intended to be limitingin any way.

It must also be noted that, as used in the specification and theappended claims, the singular form “a,” “an,” and “the” comprise pluralreferents unless the context clearly indicates otherwise. For example,reference to a component in the singular is intended to comprise aplurality of components.

Throughout this application, where publications are referenced, thedisclosures of these publications in their entireties are herebyincorporated by reference into this application to more fully describethe state of the art to which this invention pertains.

With reference to FIG. 1, a fuel cell that incorporates an ionconducting membrane of one or more embodiments of the invention isprovided. PEM fuel cell 10 includes polymeric ion conductive membrane 12disposed between cathode catalyst layer 14 and anode catalyst layer 16.Polymeric ion conductive membrane 12 includes an effective amount ofstannate as set forth below. Fuel cell 10 also includes conductiveplates 20, 22, gas channels 60 and 66, and gas diffusion layers 24 and26.

In an embodiment of the present invention, an ion conducting membranefor fuel cell applications includes an ion conducting polymer and atin-containing compound at least partially dispersed within the ionconducting polymer. In a variation, the tin-containing compound includesSn(IV). In a variation, the ion-conducting membrane further comprises ametal-containing compound having a metal (i.e., metal ion) selected fromthe group consisting of Ce(III), Ce(IV), Mn(II) and Mn(IV). Examples ofmetal-containing compounds include MnO₂, Mn₂O₃, MnCl₂, MnSO₄, CeCl₃,Ce₂(CO₃)₃, CeF₃, Ce₂O₃, CeO₂, Ce(SO₄)₂) Ce(OSO₂CF₃)₃, and combinationsthereof.

In another variation of the present embodiment, the tin-containingcompound is present in an amount from about 0.1 to about 5 weightpercent of the total weight of the ion conducting membrane.

In still another variation of the present embodiment, themetal-containing compound is a stannate. Suitable stannates includeorthostannates such as those having the following formula:M_(x)SnO₄wherein:

M is a cation; and

x is a number such that M_(x) balances the charge on SnO₄ ²⁻. Typically,x is 1 or 2 depending on M. In a refinement, M is selected from thegroup consisting of hydrogen, sodium, potassium, calcium, and magnesium.

In another refinement of the present embodiment, the stannate is ametastannate such as those having the following formula:M_(y)SnO₃wherein:

M is a cation; and

y is a number such that M_(y) balances the charge on SnO₃ ²⁻. In arefinement, M is selected from the group consisting of hydrogen, sodium,potassium, calcium, and magnesium.

In still another variation of the present embodiment, themetal-containing compound is selected from the group consisting of MnO₂,Mn₂O₃ MnCl₂, MnSO₄, and combinations thereof.

As set forth above, the membrane of the present invention includes anion conducting polymer. Such polymers include sulfonatedtetrafluoroethylene-based fluoropolymer-copolymers. Sometimes this classof polymers is referred to as perfluorosulfonic acid (PFSA) polymers.Specific examples of such polymers include the Nafion® line of polymerscommercially available from E. I. du Pont de Nemours and Company. Inanother refinement, the ion conducting polymer comprises aperfluorocyclobutyl moiety. Examples of these suitable polymers are setforth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,718,627; 2,393,967;2,559,752; 2,593,583; 3,770,567; 2,251,660; U.S. Pat. Pub. No.2007/0099054; U.S. patent application Ser. No. 12/197,530 filed Aug. 25,2008; Ser. No. 12/197,537 filed Aug. 25, 2008; Ser. No. 12/197,545 filedAug. 25, 2008; and Ser. No. 12/197,704 filed Aug. 25, 2008; the entiredisclosures of which are hereby incorporated by reference.

The following examples illustrate the various embodiments of the presentinvention. Those skilled in the art will recognize many variations thatare within the spirit of the present invention and scope of the claims.

Membrane Preparation. Sodium stannate was added at 5 wt. % based onperfluorosulfonic acid (PFSA) polymer solids in 1-propanol and water(3/2 weight ratio). Most of the stannate salt was insoluble after 24hours, and the mixture was centrifuged to remove suspended salts. Theclear supernate was coated on glass with an 8-mil coating gap, Birdapplicator, and the resultant wet film was then heated at 80° C. for 1 hand then 130° C. for 5 hours. The film was floated off glass andair-dried to obtain a 20-um membrane. Fuel cell build was with CCDMs(catalyst coated diffusion media)—0.4/0.4 mg/cm² Pt loadings, 38cm²-active areas and sub-gaskets. Operating conditions were open circuitvoltage (OCV) at 95° C., 50/50-% relative humidity anode inlets, and 5/5anode-cathode H₂/air-stoichiometries.

FIG. 2 provides plots of the cell voltage degradation and fluoriderelease rate versus time for Nafion® 1000 membrane with and withoutsodium stannate. Sodium stannate improves the durability of a Nafion®1000 membrane (by more than a factor of 2 to more than 225 h [1.8 mL H₂cross-over] from 150 h without stannate), reduced electrode voltagedegradation rates (to −180 μV/h from −550 μV/h without stannate) andreduced fluoride emissions (by a factor of 6, to 9×10⁻⁷ gF/h/cm2 from5×10⁻⁵ gF/h/cm²) using 0.4/0.4-mg/cm² Pt loadings in 38-cm² active areasof fuel cells operated at open circuit voltage at 95° C. and 50/50-%relative humidity, 5/5H₂/air-stoichiometries. The initial performance ofthe stannate treated membrane was initially lower (0.92V) than that ofthe Nafion® 1000 membrane without stannate (0.94V), but the performanceof the two membranes was equivalent after only 50 hours of fuel celloperation, primarily due to the rapid degradation of the Nafion® 1000membrane alone, without added stannate.

FIG. 3 provides plots of the RH sweep profile for Nafion® 1000 withsodium stannate. FIG. 4 provides plots of the RH sweep of Nafion® 1000with and without sodium stannate. The initial 95° C., 1.0 A/cm² fuelcell performance at less than 70% RH outlet gas streams of the Nafion®1000 membrane with sodium stannate is poorer than that of Nafion® 1000alone. However, the stannate-containing membrane is more durable by afactor of two. The fuel cell performance of the stannate containingmembrane surpasses that of Nafion® 1000 alone after 50 hours ofoperation at open circuit voltage at 95° C. and 50% RH outlet gasstreams.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. An ion-conducting membrane for fuel cellapplications, the ion-conducting membrane comprising: an ion-conductingsulfonated tetrafluoroethylene-based fluoropolymer-copolymer; and atin-containing compound at least partially dispersed within theion-conducting sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer, the tin-containing compound being present in anamount from about 0.1 to about 5 weight percent of the total weight ofthe ion-conducting membrane, the tin-containing compound being describedby formula I:M_(x)SnO₄  I M is hydrogen, sodium, potassium, calcium, or magnesium;and x is a number such that M_(x) balances the charge on SnO₄.
 2. Theion-conducting membrane of claim 1 wherein the tin-containing compoundcomprises Sn(IV).
 3. The membrane of claim 1 wherein the tin-containingcompound is a stannate.
 4. The membrane of claim 3 wherein the stannateis an orthostannate.
 5. The ion-conducting membrane of claim 1 furthercomprising a metal-containing compound comprising a metal selected fromthe group consisting of Sn(IV), Ce(III), Ce(IV), Mn(II) and Mn(IV). 6.The membrane of claim 5 wherein the metal-containing compound isselected from the group consisting of MnO₂, Mn₂O₃, MnCl₂, MnSO₄, CeCl₃,Ce₂(CO₃)₃, CeF₃, Ce₂O₃, CeO₂, Ce(SO₄)₂) Ce(OSO₂CF₃)₃.
 7. Theion-conducting membrane of claim 1 wherein the tin-containing compoundis present in an amount of about 5 weight percent of the total weight ofthe ion-conducting membrane.